Proximal intangibility not working for a rare

I just met a rare enemy in floor 3 of the Trial of Sekhema with Proximal intangibility, only this time he was invulnerable.
I stood right in his face and did 0 damage. Interestingly, none of my minions even tried to attack him either. They just grouped around him and stared like brainlets. Had to logout and in to respawn the room to progress.

I do not have the name of the mob but it was some kind of skeleton waving a thing around. Cheers